The present invention relates to a method of treating excess sludge formed in the course of purifying waste water, sewage water, etc. (hereinafter referred to as "waste water") by an activated sludge treatment.
The activated sludge method is one of several biological oxidizing treatments and is now widely employed as a method of processing waste water. According to the activated sludge method, an inlet stream of waste water is aerated in the presence of activated sludge and then the discharge from the aeration tank is fed to a settling basin to settle solids as sludge. The settled sludge is returned to the aeration tank and the effluent fluid is discharged to a stream or drain. However, according to this type of treatment the accumulation of excess sludge, i.e. additional sludge, in the final settling basin is unavoidable.
Typically, in the prior art, the excess sludge, after concentration thereof by vacuum, has been carried away by truck to be disposed of, or it has been aerobically or anaerobically digested, or after being dewatered and dried by a mechanical means, e.g. with a drying machine, it has been disposed of. However, each of these prior art methods of disposing of the excess sludge has various drawbacks.
Namely, in case the excess sludge is carried away, the volume of the sludge to be carried and disposed of, even after being concentrated with a thickener, is so enormous that the cost of handling is relatively high, since much reduction in volume cannot be expected.
On the other hand, with the method of aerobically or anaerobically digesting and decomposing the sludge with aerobic or anaerobic bacteria, a long residence period, e.g. 20-30 days, is required to reduce the volume by 30-40%. This means that a digesting tank of large volume is needed, resulting in an increase in construction cost. In addition, the digesting is seriously influenced by temperature, so precise control of temperature is required. Furthermore, even in this method, in the final stage of treatment additional means of dewatering, drying and incinerating the sludge must be employed for complete disposal of the sludge.
In the mechanical dewatering method using a centrifugal separator, which is now widely used, the filter cloth is frequently and easily clogged because the activated sludge usually contains a large amount of colloidal constituents. In addition, the water content of the resulting filter cake is usually high. Thus, it is very difficult to treat the sludge successfully by means of this method. Furthermore, since a large amount of an expensive reagent is also required for coagulation, the mechanical method is relatively costly, too, because of the high cost of the chemicals, power and work required for disposing of the filter cake.
The "activated sludge" comprises suspended solids in water containing soluble matter and colloids. The suspended solids are mainly composed of aerobic bacteria and protozoa (hereinafter referred to as "microorganisms") which spontaneously multiply if the supply of nourishment in the form of biochemically-decomposable organic matter, nitrogen and phosphorous is satisfactory.
The inventors have found, after extensive study, keeping in mind the composition of the activated sludge mentioned above, that cells of the microorganisms of which the activated sludge is mainly composed may be ruptured or destroyed by means of ultrasonic vibration, for example, to provide organic nutrients, which in turn may easily be digested by aerobic microorganisms. The inventors also found that the means of converting the microorganisms into organic nutrients include both physical and physico-chemical means. The physical means include ultrasonic vibration, homogenizer, mechanical mixer, etc. and a means of very rapidly expanding the volume of the sludge due to rapid reduction in pressure, and the physico-chemical means include, for example, decomposition by oxidizing with ozone gas.
Thus, the present invention is essentially different from the prior art method in which the digestion with aerobic or anaerobic bacteria is carried out. According to the present invention, the cells of the microorganisms are converted to provide organic nutrients.